The present invention relates generally to devices and methods for managing gas in a liquid distribution system, and more particularly to the control of gas bubbles within a capillary fluid transport and containment system.
In many applications requiring the movement or containment of fluids, the formation of bubbles of gas within the fluid can adversely affect system performance. For example, gas bubbles in the ink delivery system of an inkjet printer can degrade print quality or lead to printhead failure.
Fluids exposed to the atmosphere typically contain dissolved gases in amounts varying with the temperature. The amount of gas that a liquid can hold depends on temperature and pressure, but also depends on the extent of mixing between the gas and liquid and the opportunities the gas has had to escape. Changes in atmospheric pressure normally can be neglected since ambient atmospheric pressure stays fairly constant. However, temperature variations typically have a significant impact on the amount of gas a fluid may hold.
Most fluids exposed to the atmosphere contain dissolved gases in amounts proportional to the temperature of the fluid itself. The colder the fluid, the greater the capacity to absorb gases. If a fluid saturated with gas is heated, the dissolved gases are no longer in equilibrium and tend to diffuse out of solution. If nucleation seed sites are present along the surface containing the fluid or within the fluid, bubbles will form, and as the fluid temperature rises further, these bubbles grow larger.
Bubbles are not only composed of air, but may also include other constituents from the fluid. In an inkjet printer, for example, these include water vapor and vapors from other ink-vehicle constituents. However, the behavior of all liquids are similar, and the hotter the liquid becomes, the less gas it can hold. Both gas release and vapor generation cause bubbles to start and grow as temperature rises.
The conditions most conducive to bubble generation are the simultaneous presence of (1) generating or xe2x80x9cseedxe2x80x9d sites, (2) fluid flow and (3) bubble accumulators. These three mechanisms work together to produce large bubbles that can clog and stop flow in fluid delivery systems. When air comes back out of solution as bubbles, it does so at preferential locations, or generation or nucleation sites. Bubbles like to start at edges and corners or at surface scratches, roughness, or imperfections. Very small bubbles tend to stick to the surfaces and resist floating or being swept along in a current of fluid. When the bubbles get larger, they are more apt to break loose and move along. However, if the bubbles form in a corner or other out-of-the-way location, it is almost impossible to dislodge them by fluid currents.
While bubbles may not start at gas generating sites when the fluid is not flowing past those sites, when the fluid is moving, the bubble generation site is exposed to a much larger volume of fluid containing dissolved gas molecules. As fluid flows past the gas generating site, gas molecules can be brought out of solution to form and grow a bubble.
The third contributor to bubble generation is the accumulator or bubble trap, which can be defined as any expansion and subsequent narrowing along an fluid passage. This configuration amounts to a chamber in the fluid flow path with an entrance and an exit. The average fluid flow rate, in terms of volume of fluid per cross section of area per second, is smaller within the chamber than at the entrance or at the exit. The entrance edge of the chamber may act as a gas generating site because of its sharpness and because of the discontinuity of fluid flow over the edge. Bubbles will be generated at this site, and when they become large enough they get moved along toward the exit duct until the exit duct is blocked. Then, unless the system can generate enough pressure to push the bubble through, the fluid delivery system will become clogged and fluid delivery will be impeded.
In the field of inkjet printing, for example, there is a need to prevent air bubbles from reaching or accumulating in the inkjet printhead. Air bubble accumulation is a particular worry near a thermal inkjet printhead, which typically comprises a silicon chip containing an array of heating resistors which boil ink and expel it, through an array of orifices adjacent to the resistors and onto nearby print media. The presence of air bubbles in the printhead can seriously degrade print quality, can shorten the usable life of a printhead, and, if air accumulation results in xe2x80x9cdry firingxe2x80x9d of the printhead, can cause catastrophic failure of the printhead. This problem has typically been addressed by either xe2x80x9cwarehousingxe2x80x9d air away from the printhead, or providing active ink recirculation through the printhead to move bubbles out of the printhead.
Air xe2x80x9cwarehousingxe2x80x9d is typically used with replaceable ink cartridges where the printhead is replaced along with the ink supply (see, for example, U.S. Pat. No. 4,931,811 to Cowger et al., THERMAL INK JET PEN HAVING A FEEDTUBE WITH IMPROVED SIZING AND OPERATIONAL WITH A MINIMUM OF DEPRIMING, assigned to the assignee of the present invention). A gas accumulator is provided near the printhead nozzle plate for accumulating gas bubbles. Once the volume of gas exceeds the volume of the gas accumulator, the printhead will typically fail. Air warehousing thus necessitates increasing the size of the printhead to accommodate the gas accumulator, and is not generally suitable for long-life or permanent printheads.
Ink recirculation involves moving ink through a printhead to actively carry bubbles away from printhead. Typically used with long-life or permanent printheads, ink recirculation requires that a return path be provided from the printhead to the ink reservoir, with the attendent check valves, pumping system, and pressure regulators. Since a printer may include four or more ink colors, ink recirculation greatly increases the complexity of a printer.
The use of capillary materials in fluid containment and transport systems is well known. In the field of inkjet printing, for example, capillary foam materials are often used in ink cartridges, where the capillary strength (also referred to as capillary affinity or capillarity) of the foam can be used to provide a negative backpressure to prevent drooling of the printhead (see, for example, Baker, U.S. Pat. No. 4,771,295, THERMAL INK JET PEN BODY CONSTRUCTION HAVING IMPROVED INK STORAGE AND FEED CAPABILITY, assigned to the assignee of the present invention).
It is also known in the art to grossly vary the capillarity within a fluid system to selectively attract fluid to a region. For example, the capillarity of a porous foam ink storage member may be locally varied by compressing the foam to insure that the foam immediately adjacent to the printhead remains saturated as the cartridge is depleted (Baker, U.S. Pat. No. 4,771,295, THERMAL INK JET PEN BODY CONSTRUCTION HAVING IMPROVED INK STORAGE AND FEED CAPABILITY, assigned to the assignee of the present invention). Alternatively, the foam may be selectively compressed at the top of an ink chamber to compensate for the gravity head due to the column of ink when the pen is full (Altendorf, EP0709210, INK-JET PEN WITH CAPILLARITY GRADIENT, and related U.S. application Ser. No. 08/813715, both assigned to the assignee of the present invention).
The foams utilized in such applications, however, allow only a coarse gradation of average capillarity. When examined in detail, the fine capillary structures of such foams vary randomly over a significant range of capillary sizes, resulting in local areas within the foam where gas bubbles may become lodged. In essence, local areas of capillary widening within the foam act as minute bubble traps. Absent the application of high pressure fluid to the foam (such as may be utilized in the initial production of the pens), the volume of such foams occupied by gas increases over time, and the flow of fluid is increasingly impeded.
Similar gas management concerns exist in other fields. In fuel cells, for example, gas bubbles may be generated within the cell as the result of the chemical reaction of the reactants. Provisions must be made in the design of a fuel cell to remove these bubbles from the cell and to prevent their clogging the fluid transport paths.
There is therefore a need for passive gas management devices and methods which achieve gas management without the expense and complexity of active fluid recirculation systems.
Embodiments of the present invention comprise capillary fluid transport and containment structures in which a capillarity gradient is provided in a direction other than the primary direction of fluid transport to selectively capture and transport gas bubbles.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.